1. Field of the Invention
The present invention generally relates to an optical scanning lens, an optical scanning device and an image forming apparatus.
2. Description of the Related Art
An optical scanning device, which deflects a light flux from a light source at a uniform angular velocity by a light deflector having a deflection reflecting surface, converges the deflected light flux on a surface to be scanned as a beam spot by a scanning and image forming optical system, and, thus, scans the surface to be scanned at a uniform velocity with the beam spot, is well-known in relation to ‘image forming apparatus’ such as a digital copier, an optical printer, a laser plotter, a digital plate maker and so forth.
FIG. 1 illustrates one example of an optical scanning device.
A divergent light flux emitted by a semiconductor laser 10 is transformed into a light flux form (such as a parallel light flux or the like) suitable for subsequent optical systems by a coupling lens 12, passes through an opening of an aperture 14 so as to undergo ‘beam formation’, and is reflected by a mirror 18, while being converged in sub-scanning directions by a cylinder lens 16, and an approximately line-like image long in main-scanning directions is formed in the vicinity of a deflection reflecting surface of a rotational polygonal mirror 20. The light flux reflected by the deflection reflecting surface is incident on a scanning and image forming optical system 30 while being deflected at a uniform angular velocity as the rotational polygonal mirror 20 rotates at a uniform velocity, and is gathered in the vicinity of a surface to be scanned (actually, a photosensitive surface of a photoconductive photosensitive body or the like) 40 by a function of the optical system 30, and, thereby, a beam spot is formed on the surface to be scanned 40. By the beam spot, the surface to be scanned 40 is scanned in main scanning directions. The photosensitive surface which embodies the surface to be scanned 40 is moved in a sub-scanning direction (direction perpendicular to the plane of FIG. 1), and, the above-mentioned optical scanning is repeated. Thereby, a latent image is written on the photosensitive surface. A velocity of the above-mentioned optical scanning by a beam spot is made uniform by a function of a velocity uniformizing character of the scanning and image forming optical system 30.
Throughout the specification and claims, ‘an optical scanning lens’ is used in the above-described scanning and image forming optical system. In the example FIG. 1, the scanning and image forming optical system 30 consists of a single lens. In this case, the scanning and image forming optical system 30 itself is an optical scanning lens. When a scanning and image forming optical system consists of a plurality of optical elements (a plurality of single lenses, a lens and a concave mirror or the like), one or a plurality of single lenses used therein is an optical scanning lens.
As an optical scanning lens used in a scanning and image forming optical system, a lens obtained as a result of molding of plastic material has been used.
One problem occurring when an optical scanning lens is formed by molding plastic material is that a refractive-index distribution develops inside the thus-formed optical scanning lens.
In plastic molding, a plastic material, melted by heat, is molded by a metal die, and is cooled in the metal die. In this process, cooling of the material is fast in the periphery in comparison to the middle of the metal die. Thereby, a non-uniform distribution (the density of a fast-cooled portion is higher than the density of a slowly-cooled portion) in density and/or modification develops in the plastic. Thereby, the refractive index of the thus-formed lens is not uniform, and, thus, a refractive-index distribution develops therein.
FIGS. 2A through 2E illustrate such a refractive-index distribution. FIG. 2A shows a refractive-index distribution of an optical scanning lens 30 as a scanning and image forming optical system shown in FIG. 1 by contour lines in a section taken along a plane including the optical axis thereof and parallel to main scanning directions, and FIG. 2B shows a refractive-index distribution of that shown in FIG. 2A in directions perpendicular to the optical axis and parallel to the main scanning directions. FIG. 2C shows a refractive-index distribution of the optical scanning lens 30 by contour lines in a section taken along a plane including the optical axis thereof and parallel to sub-scanning directions, FIG. 2D shows a refractive-index distribution of that shown in FIG. 2C in directions parallel to the optical axis (axial directions), and FIG. 2E shows a refractive-index distribution of that shown in FIG. 2C in directions perpendicular to the optical axis and parallel to the sub-scanning directions.
As shown in FIGS. 2B, 2D and 2E, a refractive-index distribution in a plastic-molded lens is such that, generally, a refractive index at a peripheral portion of the lens is higher than that at a middle portion thereof.
Generally, when an optical scanning lens has such a refractive-index distribution inside thereof, actual optical characteristics thereof differ somewhat from ‘design optical characteristics of the optical scanning lens designed assuming that a refractive index therein is uniform’.
For example, when an optical scanning lens has a positive power, on average, a refractive index of a peripheral portion of the lens is higher than a refractive index of a middle portion thereof, and, such a refractive-index distribution functions to shift an actual position at which a beam spot to be formed on a surface to be scanned is formed ‘indirection in which the position goes away from a light deflector from a position determined in accordance with a design’.
A diameter of a beam spot by which an effective scanning range of a surface to be scanned is scanned changes as an image height changes depending on a curvature of field of an optical scanning lens. However, when a lens has such a refractive-index distribution therein, a diameter of a beam spot changes also due to an influence of the refractive-index distribution.
In FIG. 4, a vertical axis indicates a diameter of a beam spot and a horizontal axis indicates an amount of defocus (a difference between a position at which an image of a beam spot is formed (at which a light flux is gathered) and a position of a surface to be scanned). The vertical axis coincides with a surface of a photosensitive body as the surface to be scanned.
When an optical scanning lens has no refractive-index distribution therein and ‘a refractive index of the lens is uniform throughout the lens’, a relationship between an amount of defocus and a diameter of a beam spot is such that, as indicated by a broken line, the diameter of the beam spot is minimum at a position of a surface to be scanned (a position at which the amount of defocus is zero, actually, a position of a photosensitive body). However, when a refractive-index distribution exists, a relationship between an amount of defocus and a diameter of a beam spot is such that, as indicated by a solid line, the diameter of the beam spot at a position of a surface to be scanned is larger than that in accordance with a design (a cross point of the vertical axis and the broken line) due to ‘beam thickening’.
As materials of optical plastic lenses, mainly, acrylic resin, PC (polycarbonate) and polyolefin resin are known. Acrylic resin includes PMMA and alicyclic acrylic resin. Polyolefin resin includes ordinary polyolefin (such as polyethylene, polypropylene or the like) and alicyclic polyolefin.
FIG. 12 shows a list of optical characteristics of these resins.
A photoelasticity constant in the list of FIG. 12 can be used to determine whether double refraction of a lens formed by plastic molding is large or small. Acrylic resin is problematic because a moisture absorption is large although double refraction (photoelasticity constant) is small, and, in particular, a surface accuracy is likely to deteriorate as environment changes. Although PC (polycarbonate) has a high refractive index and a small moisture absorption, a photoelasticity constant thereof is very large and thereby double refraction is likely to develop, and wavefront aberration of a light flux transmitted thereby is likely to deteriorate.
Polyolefin resin has a small moisture absorption and a superior double refraction character. Therefore, recently, it is intended that polyolefin resin is used as a material of an optical scanning lens.
However, polyolefin resin has a relatively large mold shrinkage coefficient in comparison to other plastic materials, molding is somewhat difficult, and a refractive-index distribution is likely to develop unless molding conditions such as molding pressure, molding temperature and so forth are made to be the optimum ones.